drm/exynos: Replace DRM_LOG_KMS() by DRM_DEBUG_KMS()

There are only a few users of the DRM_LOG_KMS() macro. We can simplify
the DRM code a bit by replacing them by DRM_DEBUG_KMS().

Cc: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>